Ice cores recovered from high in the Himalayas suggest the 1990s was the warmest decade for at least 1,000 years.

The cores were drilled in a glacier flanking Xixabangma, a 8,014-metre (26,293-foot) peak on the southern rim of the Tibetan Plateau.

A one meter-long section of ice core just retrieved from a depth of 42 metres

US and Chinese scientists analysed the composition of the different layers in the ice to build up a picture of the regional climate year by year stretching back over the past millennium.

The lead researcher, Professor Lonnie Thompson, from Ohio State University, said: "This is the highest climate record ever retrieved, and it clearly shows a serious warming during the late 20th Century."

The cores are also said to show a clear record of at least eight major droughts caused by a failure of the South Asian Monsoon, the worst being a catastrophic seven-year-long dry spell that cost the lives of more than 600,000 people at the end of the 18th Century.

Volcanic activity

The international team, which also included Peruvian, Russian and Nepalese members, drilled a total of three cores in the Dasuopu Glacier over a 10-week period.

There is no question in my mind that the warming was, in part, if not totally, driven by human activity

Prof Lonnie Thompson

The seasonal layers in the ice were analysed for dust concentrations and trace chemicals, and for the ratios of the different isotopes, or types, of oxygen and hydrogen atoms.

The isotope ratios were used to extrapolate the air temperatures present when the ice was formed.

Dust concentrations were used as an indication of dryness or wetness in the region.

The presence of different chemicals such as chlorides, sulphates and nitrates were taken to be suggestive of volcanic activity, fossil fuel burning and desertification.

Human activity

The researchers write in the journal Science that their analysis shows that both the last decade and the last 50 years were the warmest in 1,000 years.

A dome protected the ice core drilling and processing operations from the elements

They say the layers covering the last century reveal a four-fold increase in dust and a doubling of chloride concentrations, suggesting an increase in dryness and desertification.

Professor Thompson said he was sure there was a clear signature of human activity in the ice layers.

"There is no question in my mind that the warming was, in part, if not totally, driven by human activity," he said.

Proxy data

Some researchers still doubt human activities are inducing global warming.

Yaks were used to carry the ice core shipping boxes to base camp

They question not only the accuracy of so-called proxy data like ice cores but some of the conclusions drawn from them - which they argue are based on debatable science.

They also point to the inconsistencies in the various temperature records.

Although surface data gathered at weather stations show a rapid warming over the last century, the atmospheric data produced in the last few decades by satellite and balloon studies show little or no warming.

The researchers believe these inconsistencies must be adequately explained before humans are blamed for climate change.

For the last 25 years, Professor Thompson and his colleagues have drilled cores from glaciers and ice caps in some of the most remote parts of the planet in an effort to recover records of ancient climate.

Most current predictions of global climate change suggest that early signs of warming will be seen at high elevations where these ice caps exist. So far, Professor Thompson's work has borne this out.